The development of in-fiber Bragg gratings (FBGs) has led to their use in wavelength measuring systems for sensor and telecommunications systems as well as for wavelength division multiplexing, dispersion compensation, laser stabilization and erbium gain flattening, all around 1550 nanometer wavelengths. These applications depend on FBG wavelength references which either do not change with temperature or change in a predictable manner.
Control of the thermal response characteristics of FBGs is important to achieve an accurate wavelength reference, or wavelength-selective passive component. Control of thermal response characteristics can provide FBGs with low temperature coefficients. Also, the temperature coefficients of FBGs can be tailored to match, or track, the temperature coefficients of other components used in an optical wavelength reference system. For example, the FBG can be used as a marker to identify a particular wavelength in the comb of wavelengths produced by a fiber Fabry Perot filter when the wavelengths of the comb drift with temperature.
Accurate wavelength referencing requires either temperature calibration or temperature compensation, or else temperature control, of the wavelength reference devices. In the latter case, thermoelectric heater/coolers requiring high electrical power consumption, generally precluding battery operation, are needed in the referencing systems. Portable test sets therefore can not easily use such temperature-controlled references. Temperature compensation or calibration is the more practical technique, with compensation being preferred since it provides an independent reference which requires no correction calculations.
Various methods have been devised for providing temperature independence for the wavelengths of FBGs. These methods range from active systems, which utilize feedback to monitor and dynamically control certain parameters, to passive devices which utilize the thermal characteristics of materials to control the sensitivity of the FBG wavelength to temperature. Passive devices are more desirable since they are much simpler and require no power source. The wavelength of an FBG is determined by the index of refraction of the fiber and the spacing of the grating, both of which change with temperature. The index of refraction dominates in the sensitivity of wavelength to temperature. However, since the index of refraction is not easily controlled, passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG. This is usually accomplished by clamping the fiber containing the FBG into a mechanical structure made of materials having different, but usually positive, temperature coefficients of expansion. The structure is arranged such that different rates of expansion between the structural members supporting the fiber result in a negative elongation of the fiber with increasing temperature. Typically the fiber is stretched at low temperatures and is allowed to relax as temperature is increased. An example of this method is described in U.S. Pat. No. 5,042,898. Another passive method of temperature compensation involves attaching the fiber containing the FBG to a material having the desired negative temperature coefficient of expansion, such as described in a paper, presented at the 22nd European Conference on Optical Communication (ECOC '96) in Oslo, by D. L. Weidman, G. H. Beall, K. C. Chyung, G. L. Francis, R. A. Modavis and R. M. Morena of Corning Incorporated, Science and Technology Division, Corning, N.Y.
The passive methods described in the preceding paragraph have the disadvantages of being relatively massive, as in the first method, or requiring very careful control of the formulation of materials to obtain the desired negative temperature coefficient of expansion, as in the second method. It is therefore an object of this invention to produce a small, simple and inexpensive device which can provide passive temperature compensation for FBGs and whose characteristics can be precisely controlled during manufacture.